So, we basically took the pans for the 2x4 outdoor sofa and extended it for one of the sofas because we wanted 4-across seating. We also finished up the look by adding vertical beams in the arms and a second 2x4 across the top (see photos). Because of the really long couch, I added extra bracing foot to make sure it won't sag.
Once complete, we determined the pillows across the back were a bit too tall, so we adjusted the plans by adding a couple beams and then lengthening it to attach to the back of the arms rather then between the arms. We pocket drilled the beams together and then braced it with 1x4 slats across the back. I also used a 1/4 router bit to soften all the edges. Anna we took your advice and pocket drilled all attachments except the seat slats for a real finished store-bought look. Cost of wood was under $100 because we went with pine. Pillows and seats from At Home put us at about $600 total for the project.